National Water-Quality Assessment (NAWQA) Project
Urban Development Results in Multiple Stressors That Can Degrade Aquatic Ecosystems by Altering the Hydrology, Habitat, and Chemistry of Streams
Results of the USGS investigation of the Effects of Urbanization on Stream Ecosystems (EUSE) found that no single environmental factor was universally important in explaining why the health of streams decline as levels of urban development increase. Across all nine metropolitan study areas, changes in the condition of the aquatic biological communities from urban development were related in varying degrees to alterations in stream hydrology, habitat, and chemistry. Even within a single study area, the three biological communities that we surveyed—algal, fish, and invertebrate—responded differently to urban development and altered environmental factors. A primary reason that the responses are different between the algal, invertebrate, and fish communities is that they have different life cycles and requirements for food, shelter, and reproduction; consequently, their responses typically vary with stressors that arise from urban-related changes in stream hydrology, habitat, and water chemistry.
Hydrology—Urban Development Leads to Increased Variability in Streamflow
Urban development typically increases the amount of water entering a stream after a storm and decreases the time that it takes for the water to travel over altered land surfaces before entering the stream. Efforts to reduce flooding by draining water quickly from roads and parking lots can result in increased amounts of water reaching a stream within a short period of time, which can lead to stream flashiness and altered stream channels. Additionally, rapid runoff reduces the amount of water available to infiltrate the soil and recharge the aquifers, which often results in lower sustained streamflows, especially during summer. Furthermore, when the hydrology of a stream is altered, the physical habitat of a stream often becomes degraded from channel erosion or lower summer flows that reduce spawning, feeding, and living spaces of the aquatic biota.
Habitat—Urban Development Can Alter Stream Channels
Urban development can result in alterations to stream habitat either directly, such as from modifications to channel and riparian areas (left), or indirectly,such as from higher streamflows that reshape the channel (right)
Stream habitats can be severely degraded where urban development occurs along the streambanks, such as where a stream has been straightened by channelization or where man-made structures have replaced natural riparian vegetation. Additionally, urban development that occurs throughout a watershed (but not necessarily directly along the streambank) can result in degraded habitat within a stream channel through flow alteration and sediment erosion. Urban development often results in deeper stream channels or an increase in the stream-channel cross-sectional area. The magnitude of these effects depends on natural environmental factors, such as the geology and soils that can influence the geomorphic characteristics of a stream and its watershed.
Chemistry—Concentrations of Contaminants in Water Increase with Urban Development
Concentrations of contaminants, including nitrogen, chloride, insecticides, and polycyclic aromatic hydrocarbons (PAHs), increased with urban development, although few measurements exceeded any human or aquatic-life benchmarks. The total concentration of insecticides increased with urban development in seven of the nine study areas. The number of individual insecticides detected, as well as the relations between these insecticide concentrations and level of urban development, varied across the study areas and appeared to coincide with regional pesticide-use patterns. For instance, higher concentrations of the insecticides chlorpyrifos and chlordane were detected in urban streams in the Atlanta, Dallas, and Raleigh study areas because of the historical use of these compounds for termite control in these areas. These results underscore the importance that regional differences need to be taken into account when comparing the influence of urban development on aquatic biota in different areas.
Aquatic Biota—Loss of Sensitive Species was the Most Consistent Biological Response to Urban Development
In this example from the Atlanta study area, urban development resulted in the loss of Ephemeroptera, Plecoptera, and Trichoptera (EPT) species, many of which are sensitive to stressors such as streamflow flashiness, habitat loss, and chemical contaminants.
Urban development generally results in a shift in the species composition of the algal, invertebrate, and fish communities. However, the most consistent change in any of the biological communities was the loss of sensitive invertebrate species and a shift to a community with a higher percentage of species more tolerant to physical and chemical stressors. A loss in the numbers of aquatic insect species that occurred in the groups Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies) (EPT), was a common response in all study areas where urban development occurred in forested watersheds. The number of EPT species is a biological-condition metric that is used in many biomonitoring programs across the country because it is sensitive to stressors from environmental degradation. A reduction of more than 50 percent of EPT species was observed in some study areas as the percentage of urban development increased in the watersheds from low to high levels.
The Invertebrate Community Begins to Degrade at the Earliest Stages of Urban Development
An often assumed response in how a biological community degrades with urban development is an initial resilience to change in biological condition over low levels of development. Then, after the community undergoes a rapid change in condition with increasing levels of urban development, an exhaustion response occurs (a "flat line" response) when only a few tolerant species are left in the community. The observed response, however, differed from this hypothetical depiction. The aquatic invertebrate communities begin to degrade with the onset of urban development, which indicates that some species are highly sensitive to physical and chemical changes associated with urban development. There was no evidence that biological communities were resilient to even low levels of urban development, based on the observation that sensitive species were being lost over the initial stages of development in relatively undisturbed watersheds. Likewise, over moderate to high levels of urban development, no exhaustion response was observed to indicate the biological community was degraded so severely that only the most tolerant species remained. Consequently, the absence of an exhaustion response indicates that stream-restoration efforts could have a rehabilitating effect on the biological condition of a stream regardless of the level of urban development in the watershed.
Regional Differences in Pre-urban Land Cover Influenced How Stream Ecosystems Responded to Urban Development
The types of land cover undergoing urban development varies among metropolitan areas, which is important because the way in which stream ecosystems respond to urban development is related to pre-urban land cover. Forest is the predominant pre-urban land cover in Boston, Portland, Salt Lake City, Birmingham, Atlanta, and Raleigh, whereas in Milwaukee, Denver, and Dallas, the predominant pre-urban land cover is some form of agriculture. Generally in watersheds converted from agricultural to urban, the streams have endured some degree of degradation prior to the initial stages of urban development. Thus, the response of stream biota to the stressors associated with urban development can appear weaker in agricultural watersheds compared to forested watersheds. For example, in comparing the aquatic invertebrate communities between the most and least developed sites in the nine study areas, the loss of EPT (mayfly, stonefly, and caddisfly) species was less in the Milwaukee, Denver, and Dallas areas than in the other six areas where pre-urban land cover is forest. The reason for this difference is not because biological communities in the agricultural regions are more resilient to stressors from urban development, but because the biological communities had already lost sensitive species to stressors from pre-urban agricultural land use activities.
Better Predictions of How Urban Development Affects Stream Health Can Be Made With Regional Models that Evaluate Multiple Stressors
In a pilot investigation using data from the Boston study area, a model was developed specifically for New England to predict how different combinations of urban-related stressors associated with stream hydrology, habitat, and chemistry affect stream health. Such tools can be used to evaluate how changes in multiple stressors can affect biological endpoints and the likelihood of attaining the desired stream-health goal. A biological condition gradient (BCG) was integrated in the model to provide a scale of stream health, which is based on the values of three biological endpoints that are sensitive to the presence and absence of certain species in the invertebrate community. The model was structured to predict the probability of attaining six tiers of stream health, which makes it possible to evaluate different management scenarios for protecting streams in urbanizing areas. For example, the model predicted that the likelihood of attaining a healthy stream would be only about 25 percent when levels of urban development exceeded 31 percent in a watershed. However, as seen in the figure, applying management actions that to reduce flashiness and improve water quality increase the likelihood of attaining a healthy stream to about 70 percent.